705 research outputs found
Inoue type manifolds and Inoue surfaces: a connected component of the moduli space of surfaces with K^2 = 7, p_g=0
We show that a family of minimal surfaces of general type with p_g = 0,
K^2=7, constructed by Inoue in 1994, is indeed a connected component of the
moduli space: indeed that any surface which is homotopically equivalent to an
Inoue surface belongs to the Inoue family.
The ideas used in order to show this result motivate us to give a new
definition of varieties, which we propose to call Inoue-type manifolds: these
are obtained as quotients \hat{X} / G, where \hat{X} is an ample divisor in a
K(\Gamma, 1) projective manifold Z, and G is a finite group acting freely on
\hat{X} . For these type of manifolds we prove a similar theorem to the above,
even if weaker, that manifolds homotopically equivalent to Inoue-type manifolds
are again Inoue-type manifolds.Comment: 36 pages, article dedicated to the 60-th birthday of Gerard van der
Gee
The Median Class and Superrigidity of Actions on CAT(0) Cube Complexes
We define a bounded cohomology class, called the {\em median class}, in the
second bounded cohomology -- with appropriate coefficients --of the
automorphism group of a finite dimensional CAT(0) cube complex X. The median
class of X behaves naturally with respect to taking products and appropriate
subcomplexes and defines in turn the {\em median class of an action} by
automorphisms of X.
We show that the median class of a non-elementary action by automorphisms
does not vanish and we show to which extent it does vanish if the action is
elementary. We obtain as a corollary a superrigidity result and show for
example that any irreducible lattice in the product of at least two locally
compact connected groups acts on a finite dimensional CAT(0) cube complex X
with a finite orbit in the Roller compactification of X. In the case of a
product of Lie groups, the Appendix by Caprace allows us to deduce that the
fixed point is in fact inside the complex X.
In the course of the proof we construct a \Gamma-equivariant measurable map
from a Poisson boundary of \Gamma with values in the non-terminating
ultrafilters on the Roller boundary of X.Comment: Minor changes that clarify some confusion have been made. Some
figures have been adde
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